This invention relates to a process for the denitration of exhaust gages produced from various types of combustion equipment such as boilers, gas turbines, engines and combustion furnaces. More particularly, it relates to an exhaust gas denitration process which is suitable for denitration purposes while the exhaust gas has a low temperature (e.g., during the starting of combustion equipment) or in a section of the exhaust gas flow path where the exhaust gas has a low temperature.
An example of a conventional exhaust gas treating system is illustrated in FIG. 3. In the system of FIG. 3, a denitrator (ammonia reduction denitrator) 2 using a catalyst is installed at the outlet of a boiler 1 in order to remove nitrogen oxides (NOx) present in the exhaust gas, and an air preheater 3 is installed at the outlet of denitrator 2 in order to lower the temperature of the exhaust gas to about 130xc2x0 C. The exhaust gas having passed through air preheater 3 is dedusted in a dust collector 4, passed through a gas-gas heater 5 and then introduced into a desulfurizer 6 where sulfur oxides (SOx) are removed therefrom. Thereafter, the exhaust gas is discharged into the atmosphere through a stack 7.
As a currently used means for the removal of nitrogen oxides present in exhaust gas, it is common practice to install a denitrator (or ammonia reduction denitrator) at the outlet of a boiler and treat the exhaust gas according to a selective catalytic reduction (SCR) process in which nitrogen oxides are decomposed into nitrogen and water vapor by using a catalyst comprising V2O5 supported on TiO2 and a reducing agent comprising NH3. On the basis of the properties of the catalyst, this denitration process requires a reaction temperature of 300 to 400xc2x0 C. so as to allow the denitrator to function properly.
Meanwhile, the temperature of exhaust gas is low during the starting of a boiler or the like, and a period of time ranging from about 20 minutes to about 2 hours is required to attain 300xc2x0 C. Accordingly, the aforesaid ammonia reduction denitrator fails to produce a sufficient denitration effect during this period. On the other hand, as a result of the recent tightening of environmental pollution controls, the nitrogen oxide concentration in exhaust gas must always be maintained at a low level. Consequently, it has become a problem to improve denitration efficiency while the exhaust gas has a low temperature as encountered during the starting of combustion equipment.
Moreover, it would be advantageous from the viewpoint of energy efficiency that highly efficient denitration can always be performed in a low-temperature section of an exhaust gas treating system.
Conventionally, many attempts have been made to perform denitration at low temperatures, but they have failed to achieve a satisfactorily high activity. After all, a reaction temperature of 300xc2x0 C. or above is required to obtain a sufficient activity.
In view of the above-described existing state of the prior art, an object of the present invention is to provide a process for the denitration of combustion exhaust gas which can remove nitrogen oxides efficiently from exhaust gas having a low temperature and which is suitable for use as a countermeasure while the exhaust gas has a low temperature (e.g., during the starting of combustion equipment) or for the denitration of a low-temperature portion of the exhaust gas.
In order to accomplish the above object, the present invention provides the following denitration processes.
According to one embodiment of the present invention, there is provided a process for the denitration of combustion exhaust gas from combustion equipment by passing the exhaust gas through an ammonia reduction denitrator, which comprises providing the exhaust gas flow path with a bypass having a low-temperature denitrator installed therein, on the downstream side of the ammonia reduction denitrator; while the temperature of the exhaust gas is not high enough to allow the ammonia reduction denitrator to function properly, as encountered immediately after the starting of the combustion equipment, passing the exhaust gas through the bypass to perform the denitration thereof by means of the low-temperature denitrator; when the ammonia reduction denitrator has come to function properly, closing the bypass to perform the denitration of the exhaust gas by means of the ammonia reduction denitrator; and regenerating the catalyst within the low-temperature denitrator while the bypass is closed.
According to another embodiment of the present invention, there is provided a process for the denitration of combustion exhaust gas from a combustion apparatus, which installing a plurality of parallel-connected low-temperature denitrators in a section of the exhaust gas flow path where the temperature of the exhaust gas ranges from room temperature to 200xc2x0 C.; and successively changing over the low-temperature denitrators in such a way that the denitration of the exhaust gas and the regeneration of the catalyst are alternately performed within each low-temperature denitrator.
In the process of the present invention, there are used one or more low-temperature denitrators which exhibit a high denitration activity in a relatively low temperature region at the initial stage of the reaction, but have a short duration of activity and which require a relatively long regeneration time. Thus, the following effects which are very valuable from a practical point of view can be achieved.
In an exhaust gas treating system incorporating a conventional ammonia reduction denitrator, a bypass is formed in the exhaust gas flow path and a low-temperature denitrator as described above is installed therein. Thus, satisfactory denitration can be performed even while the exhaust gas has a low temperature, for example, during the starting of a boiler.
Moreover, in an exhaust gas treating system in which a denitrator needs to be installed in a section having a low exhaust gas temperature, high denitration efficiency can be achieved by installing a plurality of parallel-connected low-temperature denitrators as described above.